The present invention concerns a method and apparatus for wrapping articles in plastic, whereby the cross-section of the article dictates variations in the length of plastic web required at various times, with a mechanical wrapper that revolves around the article and has at least one take-off cylinder rotating at a rate controlled in accordance with a sequence of values that depends on the article's cross-section, and releasing plastic in the length required at a particular time. The invention also concerns a device for carrying out the aforesaid method.
A device and method of the aforesaid type are known from the British patent publication No GB-OS 2 154 536. The controls that govern the rotating cylinder are not in communication with the web of plastic. They include means of constructing a model that essentially approximates the article's cross-section. The model is then employed to control the speed of the take-off cylinder. The controls are accordingly independent both of empirically detectable variations in the tension on the web and of empirically detected variations in the rate of demand. The model is a mechanically derived idealized demand that maintains the difference between the prescribed rate of supply and the momentary rate of demand as constant as possible. One drawback to the known device is that the "model," which essentially comprises several cylinders that can be shifted along one race, is very difficult to define and must be dealt with almost as a transformation of the cross-section of the article being wrapped. Furthermore, the model provides a strictly idealized structure in that the number of model parameters and hence the sequence of values being addressed can assume only a very limited number of variables.
A mechanical stretch wrapper is known from German patent publication No. DE-OS 2 750 780. It is used for wrapping articles on pallets. A strip of plastic is looped around a deflection cylinder between the supply roll and the palette. The deflection cylinder is subjected to an adjustable but constant force. When the deflection cylinder is diverted out of a middle position by increased tension on the web, controls (which involve a pressure-control valve or potentiometer for example) decrease the braking moment more or less in proportion with the angle of diversion.
This known paletted-article wrapper, however, cannot be directly employed for wrapping an article in plastic when the article's cross-section dictates variations in the length of plastic web required at various times.